III-V compound semiconductors including InP substrates have a bandgap energy corresponding to the near-infrared region and hence are studied for developing photodiodes for applications including communications, biomedical examinations, and image capturing at night.
In particular, for example, the absorption spectra of substances relating to living bodies and environments are in the near-infrared long-wavelength region. Accordingly, expansion of the absorption responsivity of the photodiodes to the long-wavelength region has been an important theme.
For example, Non Patent Literature 1 (NPL 1) proposes a photodiode having an absorption layer having an InGaAs/GaAsSb type-II multiple-quantum well structure (MQW) on an InP substrate. This photodiode has a cutoff wavelength of 2.39 μm and has a characteristic responsivity wavelength ranging from 1.7 μm to 2.7 μm. NPL 1 also proposes use of a strain-compensated structure for expanding the cutoff wavelength to a longer wavelength range, for example, 2 μm to 5 μm.
Patent Literature 1 proposes a photodiode in which a type-II (InGaAs/GaAsSb) MQW is formed on an InP substrate and zinc (Zn) serving as a p-type impurity is selectively diffused through a selective diffusion mask pattern so that pixels are formed so as to be separated by selective non-diffusion regions. On the MQW, (InGaAs diffusive-concentration-distribution-adjusting layer/InP window layer) is disposed. In this photodiode, since etching for the formation of mesa structures is not necessary, high crystallinity is maintained and dark current can be suppressed.